Particle accelerators using Linear Accelerators (LINACs) use superconducting niobium cavities to accelerate particles Niobium cavity enters into a superconductive state when it is cooled below the transient temperature (9.2 K) and the cavity is operated at cryogenic temperature using liquid helium (4.2 K) or superfluid helium (2K). A high frequency RF (radio frequency) power is coupled to the cavity using a suitable source and the charged particles are accelerated using this electromagnetic field. The cavity is tuned to RF frequency such that the cavity RF resonance frequency matches to the carrier RF supply frequency for transmitting of RF power to the charged particles. If the cavity is not in a resonating state with the RF source it would lead to reflect electromagnetic power to the source. The RF resonating frequency of the cavity depends on its physical dimensions, hence the cavity has to be expanded or compressed along the axis for optimal tuning within elastic limit. Thus, tuning mechanism is needed to maintain the efficiency of this system. Small deviations from the resonant frequency can lead to severe losses in the energy which is ultimately effectively available for particle acceleration.
Tuner is an important part of any accelerator RF cavity for static and dynamic control of its electromagnetic frequency. Functioning of a tuner is precise in case of superconducting RF cavities, where the cavity is axially compressed or expanded for changing its RF resonance frequency. In case of the SCRF cavities, quality factor is very high (Q0>109), hence it is required to have a precise control on cavity frequency to match the narrow resonance bandwidth of the order of few Hz for tuning. The main task of a tuner is to provide means to tune the cavity to its designed frequency and compensate for fast instabilities like Lorentz-force detuning (LFD), microphonics etc. Both slow and fast tuning options are required to do precise control of SCRF cavity frequency for static and dynamic loadings respectively. At present INFN blade tuner, KEK co-axial tuner, Saclay type end lever tuner etc. are currently used as tuning systems in many accelerators Laboratories around the world.
The existing systems have certain major drawbacks which are briefly described hereinafter. Hysteresis is a major problem with the existing designs of tuners and there is a constant need in this field to develop suitable tuning mechanisms having minimal hysteresis. In the current state of the art there is no such tuner design that exists which can work both as an end tuner and as a coaxial tuner. The existing tuner components are intricate in shape and their fabrication requires special machining and specially trained personnel. Tuner components require multiple welding joints for instance Blade tuner has approximately 200 joints. It is to be understood that with the increase in the number of welding joints, the method become more complex and costly. Furthermore axial as well as diametrical space requirements are high. This increases the cold mass dimensions in the cryomodule hence there is always a need for a compact device. Lastly the present tuning devices are costly. Therefore, from the disadvantages existing in the present tuning devices, it has been understood that the industry is in a demand of SCRF tuning device which is simple in construction and function and is also cost effective.
In U.S. Pat. No. 6,657,515 a tuning mechanism for a superconducting radio frequency particle accelerator cavity is disclosed, wherein the cavity comprises a number of axially aligned cells held by a frame, with at least one active cell that is axially stretchable to tune the resonant frequency of the cavity. The tuning mechanism comprises a lever arm having a center of rotation, one or more mechanical members coupling the lever arm to an active cell, and a motor adapted to move the lever arm, to thereby move the active cell through the mechanical members.
The article “The coaxial blade tuner-final report and evaluation of operation” by Bosotti et al. discloses about coaxial blade tuner based on the bending of the blades that deform from the rest position (slant of 15° respect to the central axis) to a different configuration producing an elongation (or a shortening) of the tuner itself. This deformation is generated by the rotation of the central rings with respect to the lateral ones. In order to reduce the relative rotation of the lateral rings to nearly zero, and to balance the torsional moments, the central rings rotate in opposite directions and the blades are assembled symmetrically with respect to the horizontal plane. The rotation of the central rings is obtained through an outer leverage that is directly moved by the stepping motor and, through a connecting plate, induces a displacement in opposite directions of the edges of the central rings.
In “Test operation of ball-screw-type tuner for low-loss high-gradient superconducting cavity at 77 K” by Higo et. al. the longitudinal movement of the cavity is realized by circumferential movement on a large worm wheel attached on a male screw. The slow tuning is performed by a worm gear driven by a pulse motor. This slow tuning part is mounted on a ring loosely coupled to helium vessel via twelve thin blades so that the slow tuner as a whole can be pushed fast by piezo actuator mounted on a helium vessel.
The prior art designs of the tuning systems have drawbacks that there exists no design which can work both as an end tuner or coaxial tuner, tuner components are intricate in shape and fabrication needs special machining, tuner components need multiple welding joints (e.g. Blade tuner has ˜184 joints and modified blade tuner has ˜nearly half of the number of joints). Further the axial as well as diametrical space requirement in the prior tuner devices is high which increases the cold mass dimensions in the cryomodule. There is always a need for a compact device.
Therefore, prior art in the said technical field presents a picture where there is a need for a more rugged and reliable tuning mechanism which can be fabricated at low cost and will have merits like low hysteresis, compact size and versatility.
The common elements or standard engineering elements used in existing X-link tuning device are geared motor and power screw for slow tuning movement and piezo or fast tuning actuator for rapid tuning movement. The use of these components in the present invention is optimized for better performance. The present invention consists of a tuning device for SCRF cavity, which is not only having low hysteresis but also cost effective. The presence of fast tuning system on the same actuating platform makes the device compact. The different components of the device are easy to fabricate and can easily be assembled. The geared motor of the driving mechanism is across the cavity axis, which can give accessibility to replace the motor in case of non-operation or failure of motor.